There was no increase in cancer at other stages in young women. There also was no increase in advanced disease among women older than 40.
Overall U.S. breast cancer rates have mostly fallen in more recent years, although there are signs they may have plateaued.
Some 17 years ago, Johnson was diagnosed with early-stage breast cancer at age 27, and that influenced her career choice to focus on the disease in younger women.
"Young women and their doctors need to understand that it can happen in young women," and get checked if symptoms appear, said Johnson, now 44. "People shouldn't just watch and wait."
The authors reviewed a U.S. government database of cancer cases from 1976 to 2009. They found that among women aged 25 to 39, breast cancer that has spread to distant parts of the body — advanced disease — increased from between 1 and 2 cases per 100,000 women to about 3 cases per 100,000 during that time span.
The study was published Tuesday in the Journal of the American Medical Association.
About one in 8 women will develop breast cancer in their lifetime, but only 1 in 173 will develop it by age 40. Risks increase with age and certain gene variations can raise the odds.
Routine screening with mammograms is recommended for older women but not those younger than 40.
Dr. Len Lichtenfeld, the American Cancer Society's deputy chief medical officer, said the results support anecdotal reports but that there's no reason to start screening all younger women since breast cancer is still so uncommon for them.
He said the study "is solid and interesting and certainly does raise questions as to why this is being observed." One of the most likely reasons is probably related to changes in childbearing practices, he said, adding that the trend "is clearly something to be followed."